I'm currently using an older desktop to run Plex Media Server. A lot of the time it sits idle, so I thought I'd try to mine some cryptocurrency.
I did some quick research and decided that Dogecoin would be best to get started. They can't readily be computed with ASICs so my little box could have a chance. Also, Dogecoin is sending the Jamaciaan bobsled team to the olympics after all.
I followed Dogecoin on Linux, the Complete Beginners Guide to install a wallet and a miner. I don't have a graphics card installed yet, so I'm running minerd. The only issue I had was a "duh" moment when I realized that my worker's password contained a bang ("!") and bash puked on me.
I signed up with a pool and a few days later, I'm finally at 100 Dogecoin. I'll be installing a graphics card soon and I'll see how much faster I can get my idle box to generate coins.
I'm not married to Dogecoin, so I may also switch to Litecoin or something with a better exchange rate.